1. Field of the Invention
This invention relates to a process for producing a polymer by polymerizing in a polymerization vessel a monomer having an ethylenic double bond, and particularly a process that can prevent polymer scales from adhering to polymerization vessel inner wall surfaces and others and can produce polymers having a good quality.
2. Description of the Prior Arts
As known in processes for producing polymers by polymerizing monomers in polymerization vessels, there is a problem that polymers may adhere to polymerization vessel inner wall surfaces and others in the form of scales.
Such polymer scales having adhered to polymerization vessel inner wall surfaces and others may cause a decrease in yield of polymers, a decrease in cooling capacity of polymerization vessels, and a lowering of product quality when polymer scales having adhered come off to mix into polymer products, and also may bring about a disadvantage that much labor and time must be taken to remove the polymer scales.
Moreover, since the polymer scales contain unreacted monomers, there is a possibility that operators are exposed to them to cause physical disorder.
Accordingly, in the polymerization of monomers having ethylenic double bonds, in order to prevent polymer scales from adhering to polymerization vessel inner wall surfaces and others, various methods are known in which the polymerization vessel inner wall surfaces, stirrers and so forth are coated with a polymer scale preventive agent (hereinafter xe2x80x9cscale preventive agentxe2x80x9d) by one-liquid coating to form coating films (hereinafter xe2x80x9cone-liquid coating methodxe2x80x9d). As the scale preventive agent, usable are, e.g., a polar organic compound such as an amine compound, a quinone compound or an aldehyde compound or a dye or pigment (Japanese Patent Publications (kokoku) Nos. 45-30343 and 45-30835), a polar organic compound or dye treated with a metal salt (Japanese Patent Publication (kokoku) No. 52-24953, a mixture of an electron-donating compound and an electron-accepting compound (Japanese Patent Publication (kokoku) No. 53-28347), a condensation reaction product of 1-naphthol with formaldehyde (Japanese Pre-examination Patent Publication (kokai) No. 57-164107), a condensation reaction product of a phenol compound with formaldehyde (Japanese Pre-examination Patent Publication (kokai) No. 57-192413), a polyaromatic amine (Japanese Patent Publication (kokoku) No. 59-16561), a self-condensation product of a polyhydric phenol or a self-condensation product of a polyhydric naphthol (Japanese Pre-examination Patent Publication (kokai) No. 54-7487), a condensation reaction product of a ketone resin with a phenol compound (Japanese Pre-examination Patent Publication (kokai) No. 62-236804), a condensation reaction product of an aromatic amine with an aromatic nitro compound and a material obtained by making the compound basic (Japanese Patent Publication (kokoku) No. 60-30681), and a condensation reaction product of an aromatic amine with a quinone compound (Japanese Pre-examination Patent Publication (kokai) No. 61-7309).
In the case of polymer scale preventive coating films obtained by such one-liquid coating methods, scales tend to adhere to the vicinity of a gas-liquid boundary surface in the polymerization vessel during polymerization, or, depending on the composition of a polymerization reaction mixture, scales tend to adhere to the whole wall surface. Accordingly, to prevent this, it is known to mix in a coating liquid containing the scale preventive agent a water-soluble polymeric compound such as an anionic polymeric compound, an amphoteric polymeric compound, a cationic polymeric compound or a hydroxyl-group-containing polymeric compound; an inorganic colloid; or a substance having no affinity for monomers, as exemplified by an inorganic salt such as an alkali metal salt (hereinafter xe2x80x9cscale preventive auxiliary agentxe2x80x9d). These one-liquidcoating methods are effective for preventing the adhesion of polymer scales when monomers having ethylenic double bonds are polymerized in polymerization vessels.
In instances where no sufficient polymer scale prevention effect can be obtained by the one-liquid coating method, a method of forming coating films by coating two types of coating liquids through two stages (hereinafter xe2x80x9ctwo-liquid two-stage coating methodxe2x80x9d) is proposed, which comprises a) coating a coating liquid containing the scale preventive agent as described above, to form a first layer, and b) coating further thereon a coating liquid containing the above polymer scale preventive auxiliary agent, to form a second layer (Japanese Pre-examination Patent Publication (kokai)Nos. 3-74404, 2-80403, 2-80402, 2-80401 and 2-47102).
In both the above one-liquid coating method and two-liquid two-stage coating method for preventing the adhesion of polymer scales, spray coating is usually used as a coating process in view of productivity including operability.
In the one-liquid coating method of coating the scale preventive agent by spray coating, the coating film is formed by a process comprising the following steps 1 to 3.
Step 1: A coating liquid containing the scale preventive agent is coated on the polymerization vessel inner wall surface and other surfaces with which monomers come into contact.
Step 2: The coated surfaces are dried to form a dry film.
Step 3: The surface of the coating film thus formed is washed to remove any excess coating liquid.
In the two-liquid two-stage coating method comprising coating the scale preventive agent and coating the polymer scale preventive auxiliary agent both by spray coating, the coating film formation comprising the same steps 1 to 3 as the above is operated also in the second-stage coating.
When the above spray coating is used, the surfaces of baffles and stirring blades that face polymerization vessel inner wall surfaces stand within the dead angle from a spray nozzle. Since it is hard for the coating liquid to reach the surfaces of such portions standing blind or hidden from the spray nozzle, the scale preventive agent can not be coated thereon in the same way as on the surfaces not standing blind. Thus, it is difficult to form a uniform coating film over the surfaces standing blind and the surfaces not standing blind. If a coating film in a quantity effective enough to prevent the adhesion of polymer scales is intended to be formed also on the blind surfaces, it is inevitable to use a coating liquid containing the scale preventive agent in a larger quantity than that for the other surfaces. It follows that an unnecessarily excess preventive agent is applied on the surfaces not standing blind. Hence, the coating film thus formed have had an uneven coating thickness and the coating film have had a larger thickness locally than is necessary.
The formation of polymer scale preventive coating films by spray coating has also had the following problems.
(1) Usually, the coating film comprising the scale preventive agent is formed previously for each polymerization batching. Since it is common for the scale preventive agent to have a color, the scale preventive agent is repeatedly coated as the polymerization is batched repeatedly in a larger number, so that the coating film may have a large thickness at some part. The part having such a thick coating film may come off to become included into the reaction mixture, or the scale preventive agent may be coated on polymer scales having already adhered to the polymerization vessel inner wall surfaces and others and may come off together with a part of the scales to mix into the resultant polymerization products. This may cause colored particles or fish eyes brought in their formed products or may cause a low product quality such as a high initial discoloring of formed products, disadvantageously.
(2) As stated above, the effect of preventing scales at the surfaces standing blind or hidden in the polymerization vessel, standing within the dead angle from the spray nozzle, can not be said to be so much sufficient, considering the scale preventive agent applied in a fairly larger quantity than that on other surfaces.
(3) The spray coating requires a drying step of drying the coated surfaces, and takes a time necessary for forming the coating film of the scale preventive agent. Accordingly, in respect of an improvement of productivity, it is sought to shorten the time necessary for forming the coating film.
As a measure for eliminating the above disadvantages in the spray coating, a method is proposed in which a coating liquid containing a scale preventive agent is coated using steam as a carrier (hereinafter xe2x80x9cwsteam coatingxe2x80x9d) (Japanese Patent Publication (kokoku) No. 1-5044). As the coating liquid in this method, used is a coating liquid comprised of the scale preventive agent alone or a coating liquid to which the scale preventive auxiliary agent is further added.
This steam coating has the following advantages.
(1) A thin and uniform coating film of the scale preventive agent, necessary for preventing the adhesion of scales effectively can be formed using the coating liquid in a small quantity.
(2) The coating film of the scale preventive agent, necessary for achieving the scale prevention effect can be formed using the coating liquid in a small quantity, also on the portions standing blind or hidden in the polymerization vessel, standing within the dead angle from the spray nozzle. Thus, the polymer scale prevention effect can be attained also on these portions.
(3) The drying step is unnecessary in the coating film forming step, so that the time necessary for forming the coating film of the scale preventive agent can be shortened.
Incidentally, in the steam coating, the coating liquid and steam are mixed so that the coating liquid is carried by the steam and can be applied to the polymerization vessel inner wall surfaces and others. Accordingly, the concentration of the scale preventive agent in the coating liquid is set taking account of the fact that the solution is diluted with steam. Usually, the concentration of the scale preventive agent in the coating liquid for steam coating is set 4 to 40 times that of the one for spray coating, although the amount of a scale preventive agent necessary in steam coating is approximately equivalent to that necessary in spray coating.
In contrast to the advantages, the steam coating has problems on the following points.
(1) Although the steam coating enables uniform coating in a polymerization vessel, the deposition of scale can be prevented insufficiently around the interface between the gas-liquid phases.
(2) As the result of the insufficient prevention of scale deposition around the interface between gas-liquid phases, the polymer scale deposition will grow around the interface with repetition of polymerization runs. A part of the grown deposited scale may peel off the inner surfaces of the polymerization vessel during polymerization and be incorporated into a polymer product to cause formation of fisheyes.
(3) A scale preventive agent is coated on the inner surfaces of a polymerization vessel repeatedly as polymerization runs are repeated. Consequently, the layer of the scale preventive agent become thicker gradually. A part of the thick layer of the agent may peel off during polymerization and be incorporated into polymer products to cause colored particles. The colored particles will lower anti-initial discoloration properties, particularly luminosity index L, of polymer products.
An object of the present invention is to provide a process for producing a polymer by polymerizing a monomer having an ethylenic double bond, which can shorten the time for forming coating films of scale preventive agents to improve productivity, can improve the effect of preventing scales, can make colored particles less mix into polymer products obtained by this process, can lessen fish eyes and initial discoloring of formed products and can improve the quality of polymeric products and their formed or molded products.
The above subject can be settled by a process for producing a polymer by polymerizing in a polymerization vessel a monomer having an ethylenic double bond, wherein
the polymerization vessel has a polymer scale preventive coating film on its inner wall surfaces and other surfaces with which the monomer comes into contact during polymerization;
the coating film being formed by coating a first coating liquid containing a compound selected from the group consisting of an aromatic compound having 5 or more conjugated  bonds and a heterocyclic compound having 5 or more conjugated  bonds and a second coating liquid containing at least one hydrophilic compound selected from the group consisting of a water-soluble polymeric compound, an inorganic colloid, an inorganic salt and an acid; the first coating liquid and the second coating liquid being simultaneously coated by means of steam as a carrier.
According to the polymerization process of the present invention, the time for forming coating films of scale preventive agents can be shortened to improve productivity, and also, when monomers having an ethylenic double bond are polymerized, polymer scales can be prevented effectively from adhering to not only wall surfaces at the liquid-phase portion in the polymerization vessel but also stirrers, baffle surfaces facing the wall surface, and the vicinity of the boundary surface between the gaseous phase and the liquid phase. Hence, the quality of polymer products can be improved and the colored particles can be made very less mix into polymers, and also formed products obtained by forming the polymers into sheets can be made to have very less fish eyes and also have good anti-initial discoloring.